1. Field of the Invention
The present invention relates to a magnetic recording medium. More particularly, it relates to a magnetic recording medium with good dispersibility of fine magnetic powder in a binding agent and improved electrical properties.
2. Related Art
Generally, a magnetic recording medium comprises a substrate film and a magnetic layer coated thereon by applying a mixture containing magnetic powder, a binding agent, an organic solvent and other suitable components followed by drying.
A recently used magnetic recording medium such as a video tape is required to have improved playback output and S/N ratio so as to create a sharp picture and/or clear sound in order to accommodate a video tape recorder which seeks highly densified recording. To this end, the particle size of the fine magnetic powder to be contained in the magnetic layer is increasingly reduced to provide a specific surface area of 30 m.sup.2 /g or larger measured by the BET method.
Since the surface of such fine magnetic powder is hydrophilic, it is difficult to disperse the magnetic powder in a lipophilic binding agent. In addition, the magnetic powder tends to coagulate and adsorb more liquid as its particle size becomes smaller, which results in an increase of viscosity of the mixture so that sufficient shear is not effectively applied to the mixture in the dispersing equipment. Therefore, the dispersability of the magnetic powder deteriorates and, in turn, the playback output and S/N ratio cannot be sufficiently improved.
To improve the dispersibility of the fine magnetic powder in the magnetic layer mixture, it is proposed to use, as a dispersing agent, a saturated or unsaturated fatty acid having a hydrophilic group at one end and a lipophilic group at the other end (cf. Japanese Patent Publication Nos. 18221/1969 and 9488/1983). Such a fatty acid has weak bonding strength with the magnetic powder so that it cannot improve the dispersibility of the fine magnetic powder in the binding agent and thus the playback output and S/N ratio are not desirably improved. In addition, the magnetic layer containing the particularly fine magnetic powder tends to be damaged and its abrasion resistance deteriorates when slid against a magnetic head. Therefore, the abrasion resistance is improved by the addition of a filler with a Moh's hardness of at least 5 such as Cr.sub.2 O.sub.3 and Al.sub.2 O.sub.3. However, the dispersibility of such a filler is worse than that of the magnetic powder above and is difficult to be dispersed in the binding agent, so that the electromagnetic conversion characteristics are not sufficiently improved by the improvement of the surface flatness of the magnetic layer.